Fiber-reinforced resin materials, or “composite materials” as they are commonly known, offer a number of advantages over conventional metal materials including high strength-to-weight ratios and good corrosion resistance. Conventional composite materials typically include glass, carbon, or polyaramide fibers in woven and/or non-woven configurations. In the raw material stage, the fibers can be pre-impregnated with resin or left dry. If dry, the fibers can be infused with resin after lay-up on a mold surface. Heat and/or pressure can be applied to the resin-impregnated fibers on the mold surface to cure the resin and harden the laminate in the shape of the mold. The heat and pressure can be applied with an oven, an autoclave, a heated flat or contoured forming tool, or a combination of methods including the use of a vacuum bag.
Composite parts can be formed in the above manner on both male and female tools. With male tools, the fiber plies are applied to an exterior mold surface that forms an inner mold line of the part. Adding plies to the lay-up on a male tool increases the thickness of the part and changes the outer mold line, but the inner mold line remains unchanged. Conversely, with female tools, the fiber plies are applied to an interior mold surface that forms an outer mold line of the part. Adding plies to the lay-up on a female tool increases the thickness of the part and changes the inner mold line, but the outer mold line remains unchanged.
Female tools are desirable when the mating surface is located on the exterior of a part because female tools allow the outer mold line (i.e., the exterior surface) to be tightly controlled. Female tooling (also known as “outer mold line tooling”) is also desirable when making multiple parts having the same external dimensions but different thicknesses. Aircraft fuselages, for example, often have multiple frames with the same external dimensions but different thicknesses. In this situation, all of the frames can be made with a single female tool because the tool allows the thickness to vary without changing the external dimensions. If future growth of the aircraft requires further thickening of the frames, this can be achieved without changing tooling. Conversely, if male tooling were used, then a separate tool would be required for each different frame thickness.
One problem that arises when manufacturing composite parts with female tooling, however, is that the fiber plies tend to bridge and/or wrinkle across internal radii on the mold surface. FIG. 1, for example, illustrates a cross-sectional end view of fiber material 110 laid up on a portion of a female tool 102 in accordance with the prior art. The female tool 102 includes an interior mold surface 104 having a first side region 103 spaced apart from a second side region 105 by a radius region 106. A vacuum bag 120 is positioned over the fiber material 110 and evacuated to compress the fiber material 110 against the mold surface 104. As the vacuum bag 120 is being evacuated, the outside air pressure presses the fiber material 110 firmly against the side regions 103 and 105, resisting movement of the fiber material 110 into the radius region 106. This resistance causes the fiber material 110 to bridge across the radius region 106, thereby reducing the fiber density in this region. The reduction in fiber density in this region can compromise the structural integrity of the finished part.